cell internally, usually by changing their conformation or permitting a molecule to enter a channel that is part of the receptor. The next part is transduction. This is the process by which there are relay molecules that often involves the addition or subtraction of phosphate groups from larger molecules in what involves multiple steps. The last part is called the “response phase”. It can involve any possible cellular activity that is present in the body, triggered by the transduction phase. The response phase can involve the turning on or turning off of specific genes. Cellular communication can happen locally or from a distance. Locally, there can be communication via gap junctions between local cells. Plants themselves are connected through plasmodesmata. These kinds of communications are seen in embryonic development. Cell communication happens locally through paracrine signaling, which is signaling that happens over short distances. Autocrine signaling happens when cells act on identical cells. Endocrine signaling happens over large distances within the organism. All of these types of signaling mechanisms can happen in both plants and animals.
TISSUE DIFFERENTIATION Cells that are of the same type form tissues, which together form a specific function. In this case, we are talking about multicellular animal organisms. There are four types of tissues in the human body, each of which forms a different function. These are connective tissue, epithelial tissue, nervous tissue, and muscle tissue. In general, connective tissue is supportive to many other tissue types, while epithelial tissue creates protective barriers and is involved in ion and molecule diffusion. Nervous tissue will both transmit and integrate information and muscle tissue initiates movement. Epithelial tissue is the tissue that forms glands. It forms the coverings over body surfaces and lines body cavities. The tissues that form the receptors for the special senses (like smell and taste) is also epithelial tissue. These cells are closely connected to one another and have junctions that connect each other. These are cells that have an 38
